The present invention relates to a laser heterodyne interferometric method and system for measuring ultrasonic displacements. The invention is particularly directed toward measuring in a reliable and convenient manner the small displacements of the free surface of a workpiece subjected to ultrasound.
In present industry, quality control is very important. Among the different techniques used for testing manufactured products, ultrasonic methods are the most suitable. These are generally based on the launching of an ultrasonic transient wave inside the material to be tested using a piezoelectric transducer brought into contact with the material and receiving by means of the same transducer an echo coming from a surface of the material or from within the sample itself. This echo may be indicative of a defect inside the sample or in the case of a defect-free material where the echo (or echo sequence) is produced by a back surface of the sample, such echo may be used to derive information on properties of the material, such as grain size, porosity, different phases and residual stresses. In practice, the ultrasonic wave produced by the transducer may not be uniform or may have an inappropriate time variation which will strongly affect the accuracy of the ultrasonic inspection.
The surface deformations or displacements of a material produced by an ultrasonic piezoelectric transducer occur in the range of frequencies extending from 0.5 MHz to 50 MHz at most. High ultrasonic frequencies are generally strongly attenuated by currently used engineering materials so the frequencies of interest generally do not exceed 15 MHz. The displacements are generally much less than an optical visible wavelength (5000 .ANG.) and range from a fraction of 1 .ANG. to a few hundred .ANG. at most.
In order to check or characterize a piezoelectric transducer or to obtain a quantitative measurement of the surface displacement of a workpiece subjected to ultrasound, an optical interferometric probe is advantageously used. A known design is basically a Michelson interferometer which senses directly the surface displacement since one of the mirrors of the interferometer is constituted by the surface itself. The light source is a laser which produces a nearly collimated beam. Since the surface is not generally polished but is rough, good contrast interference fringes are only observed when the surface is set at the focus of a lens. Such a system is limited in use for best detection conditions to only one of the speckle spots scattered by the surface. Therefore, in practice, most of the incident light intensity is wasted and depending upon the surface scattering properties and the lens numerical aperture, only a small fraction of the intensity is used for interference. As a result, such an interferometric probe may lack adequate sensitivity.
It is also important to note that in the above interferometric probe the phase difference between the two interferring beams and therefore the detected signal are strongly affected by ambient vibrations which occur at frequencies mostly in the audio-range (&lt;100 KHz) but with amplitudes which can exceed one optical wavelength. Various solutions have already been proposed to solve this problem in both the homodyne interferometric probes (in which the optical frequency is the same in both arms of the interferometer) and heterodyne interferometric probes (in which the optical frequency in one arm has been shifted by f.sub.B by means of a Bragg or acoustooptic cell and the interference signal appears at the shift frequency f.sub.B).
For instance, in several homodyne interferometric probes, the path length change caused by ambient vibrations is compensated by moving the reference mirror. The error signal used for compensation can be obtained by dithering the reference mirror and phase detection or by finding the reference voltage level corresponding to maximum sensitivity. Such systems which rely on active stabilization do not work well in an industrial environment where the vibration level is high and are limited to laboratory experimentations. Using a different design called quadrature-dual interferometer, where the Michelson interferometer is slightly modified by adding a birefringent plate, and by using two detectors, it is possible to derive a signal independent of ambient vibrations. In this system, one detector records a signal varying in proportion to the sine of the optical path difference whereas the other measures a signal varying as its cosine. By squaring these two signals, the sensitivity to ambient vibrations is removed. This system has the drawback of requiring to square a signal at a frequency of a few MHz which is in practice difficult. Another drawback is the use of two detectors which should be adjusted to see the same part of the fringe pattern and have their outputs amplified to the same level; thus, any slight change in the set-up will affect the operation.
Several heterodyne interferometric systems have been previously described and two commercial versions are known to exist. In such systems, the ultrasonic displacement produces a phase modulation of the optical beam impinging upon the probed surface. After mixing with the beam which is reflected by the reference mirror and whose frequency is shifted by f.sub.B, f.sub.B being much larger than the ultrasonic frequencies involved, a varying intensity signal I.sub.D called the fringe signal is detected at the output of the interferometer and is given by the following relation: ##EQU1## Where I.sub.L is the laser intensity;
R is the effective transmission coefficient in intensity for the reference beam; PA1 S is the effective transmission coefficient in intensity for the beam reflected off the surface (S is much less than unity); PA1 f.sub.B is the shift frequency; PA1 .delta..sub.s (t) is the surface displacement as a function of time t; PA1 .lambda. is the optical wavelength; and PA1 .phi.(t) is a phase factor which depends upon the interferometer path difference and is affected by ambient vibrations. PA1 (a) generating a laser beam having a predetermined intensity; PA1 (b) dividing the laser beam into first and second beam portions having respective intensities representing minor and major fractions of the predetermined intensity, the first beam portion being angularly displaced relative to the second beam portion and being frequency shifted by a predetermined frequency; PA1 (c) passing the second beam portion through an optical lens off-center thereof to focalize the second beam portion onto the free surface of the material subjected to ultrasound, thereby scattering same; PA1 (d) combining the scattered second beam portion with the first beam portion to obtain an optical fringe signal; PA1 (e) converting the optical fringe signal into an electrical fringe signal comprising a central peak at the predetermined frequency and a sideband on either side of the central peak; and PA1 (f) processing the electrical fringe signal through circuitry means without demodulating a phase modulation produced by ambient vibrations, to extract a signal proportional to the displacement of the free surface. PA1 an optical lens disposed to receive off-center thereof the second beam portion for focalizing the second beam portion onto the free surface of said material subjected to ultrasound, thereby scattering same;
Since .delta..sub.s (t) is much less than .lambda., one can write: ##EQU2## which shows that the ultrasonic displacement causes a weak sideband on either side of a central peak at the shift frequency f.sub.B. The absolute value of the displacement can be readily determined from the magnitude of the side bands compared to that of the central peak; this is an advantage of an heterodyne interferometric probe over an homodyne one for which a real time absolute measurement is not directly possible.
A commercially available heterodyne interferometric probe manufactured by the Disa Company is limited to frequencies below 1 MHz and relatively large displacements. Its field of application is actually the measurement of ambient vibrations. On the other hand, in the heterodyne interferometric probe developed by the Nondestructive Testing Centre of the Atomic Energy Research Establishment (Harwell) in England, ambient vibrations are continuously compensated by varying the shift frequency f.sub.B in such a way that the overall phase 2.pi.f.sub.B t+.phi.(t) is unaffected by the vibrations. However, such a probe is complex and very expensive.
Several systems are also known which use a VHF receiver tuned to one of the side bands of the fringe signal. They are insensitive to ambient vibrations but are limited to only continuous ultrasonic displacements, and the phase of the displacement is lost.